Ragnhild Bieltvedt Skeie

Climate forcing from the transport sectors

Although the transport sector is responsible for a large and growing share of global emissions affecting climate, its overall contribution has not been quantified. We provide a comprehensive analysis of radiative forcing from the road transport, shipping, aviation, and rail subsectors, using both past- and forward-looking perspectives. We find that, since preindustrial times, transport has contributed ≈15% and 31% of the total man-made CO2 and O3 forcing, respectively. A forward-looking perspective shows that the current emissions from transport are responsible for ≈16% of the integrated net forcing over 100 years from all current man-made emissions. The dominating contributor to positive forcing (warming) is CO2, followed by tropospheric O3. By subsector, road transport is the largest contributor to warming. The transport sector also exerts cooling through reduced methane lifetime and atmospheric aerosol effects. Shipping causes net cooling, except on future time scales of several centuries. Much of the forcing from transport comes from emissions not covered by the Kyoto Protocol.

Global premature mortality due to anthropogenic outdoor air pollution and the contribution of past climate change

Increased concentrations of ozone and fine particulate matter (PM2.5) since preindustrial times reflect increased emissions, but also contributions of past climate change. Here we use modeled concentrations from an ensemble of chemistry?climate models to estimate the global burden of anthropogenic outdoor air pollution on present-day premature human mortality, and the component of that burden attributable to past climate change. Using simulated concentrations for 2000 and 1850 and concentration?response functions (CRFs), we estimate that, at present, 470?000 (95% confidence interval, 140?000 to 900?000) premature respiratory deaths are associated globally and annually with anthropogenic ozone, and 2.1 (1.3 to 3.0) million deaths with anthropogenic PM2.5-related cardiopulmonary diseases (93%) and lung cancer (7%). These estimates are smaller than ones from previous studies because we use modeled 1850 air pollution rather than a counterfactual low concentration, and because of different emissions. Uncertainty in CRFs contributes more to overall uncertainty than the spread of model results. Mortality attributed to the effects of past climate change on air quality is considerably smaller than the global burden: 1500 (?20?000 to 27?000) deaths yr?1 due to ozone and 2200 (?350?000 to 140?000) due to PM2.5. The small multi-model means are coincidental, as there are larger ranges of results for individual models, reflected in the large uncertainties, with some models suggesting that past climate change has reduced air pollution mortality.

Comparing the climate effect of emissions of short- and long-lived climate agents

Multi-gas climate agreements require a metric by which emissions of gases with different lifetimes and radiative properties can be placed on a common scale. The Kyoto Protocol to the United Nations Framework Convention on Climate Change uses the global warming potential (GWP) as such a metric. The GWP has attracted particular criticism as being inappropriate in the context of climate policy which seeks to restrict warming below a given target, because it gives equal weight to emissions irrespective of the target and the proximity to the target. The use of an alternative metric, the time-dependent global temperature change potential (GTP), is examined for its suitability and the prospects for it including very short-lived species. It retains the transparency and relative ease of use, which are attractive features of the GWP, but explicitly includes a dependence on the target of climate policy. The weighting of emissions using the GTP is found to be significantly dependent on the scenarios of future emissions and the sensitivity of the climate system. This may indicate that the use of any GTP-based weighting in future policymaking would necessitate regular revisions, as the global-mean temperature moves towards a specified target.

Costs and global impacts of black carbon abatement strategies

Abatement of particulate matter has traditionally been driven by health concerns rather than its role in global warming. Here we assess future abatement strategies in terms of how much they reduce the climate impact of black carbon (BC) and organic carbon (OC) from contained combustion. We develop global scenarios which take into account regional differences in climate impact, costs of abatement and ability to pay, as well as both the direct and indirect (snow-albedo) climate impact of BC and OC. To represent the climate impact, we estimate consistent region-specific values of direct and indirect global warming potential (GWP) and global temperature potential (GTP). The indirect GWP has been estimated using a physical approach and includes the effect of change in albedo from BC deposited on snow. The indirect GWP is highest in the Middle East followed by Russia, Europe and North America, while the total GWP is highest in the Middle East, Africa and South Asia. We conclude that prioritizing emission reductions in Asia represents the most cost-efficient global abatement strategy for BC because Asia is (1) responsible for a large share of total emissions, (2) has lower abatement costs compared to Europe and North America and (3) has large health cobenefits from reduced PM10 emissions.

Aerosol single-scattering albedo over the global oceans: Comparing PARASOL retrievals with AERONET, OMI, and AeroCom models estimates

The aerosol single-scattering albedo (SSA) over the global ocean is evaluated based on polarimetric measurements by the PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) satellite. For the first time, global ocean SSA and Absorption Aerosol Optical Depth (AAOD) from this instrument are shown and evaluated against other observations (the Aerosol Robotic Network, AERONET, and the Ozone Monitoring Instrument, OMI). The observational data sets compare reasonably well, with the majority of the colocated points within 0.05 of the AERONET measurements. PARASOL shows that SSA is characterized by high spatial and seasonal variability, also over the open ocean far from the inland emission regions. The near global coverage in the visible spectral range provided by the PARASOL retrievals represents a unique opportunity to evaluate aerosol optical properties simulated by global aerosol models, as performed in the Aerosol Comparisons between Observations and Models (AeroCom) framework. The SSA (AAOD) estimated by the AeroCom models is generally higher (smaller) than the SSA (AAOD) retrieved from PARASOL. On the other hand, the mean simulated aerosol optical depth is consistent or slightly underestimated compared with observations. An overestimate of the aerosol scattering, compared to absorption, by the models would suggest that these simulate an overly strong aerosol radiative cooling at top of atmosphere, over most of the ocean surfaces. This implies that aerosols have a potentially stronger direct and semidirect impact within the atmosphere than currently simulated.

Forty-seven years of weekly atmospheric black carbon measurements in the Finnish Arctic: Decrease in black carbon with declining emissions

Concentrations of atmospheric black carbon, [BC], were determined from filter samples collected weekly at Kevo, Finland (69°45′N, 27°02′E), from 1964 to 2010 using optical and thermal optical methods. The data provide the longest record of directly measured [BC] in the Arctic. The mean winter, spring, summer, and autumn [BC] based on the entire data set were 339, 199, 127, and 213 ng m−3, respectively. Annual mean [BC] decreased from ~300 in ~1970 to 82 ng m−3 in 2010. [BC] data sets from other Arctic sites show similar trends, but concentrations at Kevo are generally higher. From ~1970 to 2010 the [BC] decreased by ~1.8% yr−1. However, [BC] did not decrease monotonically. Instead, cyclical peaks occurred around 1976–1977, 1985–1987, and 1999. During such periods, nickel concentrations were well correlated with [BC]. This suggests that emissions from extensive ore smelting on the Kola Peninsula were significant contributors of particulate matter observed at Kevo. Simulations of [BC] at Kevo using the OsloCTM3 model using different emission inventories and meteorological data sets were performed. Modeled concentrations were lower than observed by a factor of 4. The results indicated that circulation changes can explain year to year variability, but the downward trend in the observations is mostly explained by emissions. Emission inventories in Europe, Russia, and the former Soviet Union are poorly constrained and appear to need revision in order to match observed trends in BC atmospheric concentrations.

Dynamical processes related to cyclone development near Greenland

An unusual cyclone that moved over Greenland and caused blizzard conditions over eastern Greenland and northern Iceland on 20-21 September 2003 is investigated. Numerical simulations are conducted to assess the role of Greenlands orography for the development, as well as to evaluate the significance of other factors such as latent heating and sea surface temperature (SST). The simulations reveal that the cyclone evolution was crucially dependent on an interaction between the background flow and the orography of Greenland. When the orography is removed, a deep, well organized baroclinic low develops rapidly and moves eastward at 75°N. Conversely, in the control run, which is similar to the analyses, the evolution of the (primary) baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. Instead, a secondary low developing off Greenlands east coast at 68°N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. This secondary low, absent in the run without orography, then moves eastward and causes the extreme weather conditions that were observed. Inversion of selected potential vorticity anomalies lends support to the above explanation. Further sensitivity experiments show that latent heating contributes about half of the deepening of the low, while SST amounts contribute much less.

Comparison of aerosol optical properties above clouds between POLDER and AeroCom models over the South East Atlantic Ocean during the fire season

Aerosol properties above clouds have been retrieved over the South East Atlantic Ocean during the fire season 2006 using satellite observations from POLDER (Polarization and Directionality of Earth Reflectances). From June to October, POLDER has observed a mean Above-Cloud Aerosol Optical Thickness (ACAOT) of 0.28 and a mean Above-Clouds Single Scattering Albedo (ACSSA) of 0.87 at 550 nm. These results have been used to evaluate the simulation of aerosols above clouds in five Aerosol Comparisons between Observations and Models (Goddard Chemistry Aerosol Radiation and Transport (GOCART), Hadley Centre Global Environmental Model 3 (HadGEM3), European Centre Hamburg Model 5-Hamburg Aerosol Module 2 (ECHAM5-HAM2), Oslo-Chemical Transport Model 2 (OsloCTM2), and Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS)). Most models do not reproduce the observed large aerosol load episodes. The comparison highlights the importance of the injection height and the vertical transport parameterizations to simulate the large ACAOT observed by POLDER. Furthermore, POLDER ACSSA is best reproduced by models with a high imaginary part of black carbon refractive index, in accordance with recent recommendations.

Reply to 'Interpretations of the Paris climate target'

Millar et al. reply — Our paper aimed to remain as consistent as possible with the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) definitions that have informed the United Nations Framework Convention on Climate Change (UNFCCC) negotiations. The definition of global average temperature in the Paris Agreement is undoubtedly important, and different interpretations are possible, as acknowledged in our paper. However, the Paris Agreement built on the reports of Working Group I and II1,2 of the IPCC AR5. In these reports, global temperature change was explicitly defined using the observations in the period 1850–1900 as “an approximation of preindustrial levels” (Fig. 1 of ref. 2). Climate model projections were assessed relative to 1986–2005 and then expressed relative to 1850–1900 using observed warming between these periods in the HadCRUT4 dataset3 (+ 0.61 °C). Based on the IPCCAR5-assessed4 near-term projections of a warming of 0.3–0.7 °C for the period 2016–2035 compared to 1986–2005, warming in the decade 2010–2019 is expected to be centred on 0.93 °C above 1850–1900, given forcing consistent with the representative concentration pathways and no large volcanic eruptions. Such a level of warming is consistent with the “increase of 0.85 °C [to 2012] since 1880, a good approximation for pre-industrial levels” reported in the Structured Expert Dialogue (SED)5 — dashed light blue line in Fig. 2 of Schurer et al.6 — and with the independent estimate for 2015 human-induced warming used in our paper7. Alternative definitions of global average temperature or preindustrial conditions may not be consistent with “observed impacts of climate change at 0.85 °C of warming”5 (original emphasis) in the context of which the UNFCCC longterm temperature goal was agreed — over the period 2006–2015, warming (relative to 1850–1900) in datasets that stretch back to 1850 are: 0.84 °C (HadCRUT4), 0.92 °C (HadCRUT4: Cowtan and Way) and 1.00 °C (Berkeley Earth). We aimed to remain as consistent as possible with the IPCC-AR5 definitions that have informed the UNFCCC negotiations. We therefore proposed 0.6 °C of warming above the average of the present decade as “a possible interpretation of ‘pursuing efforts to limit the temperature increase to 1.5 °C’ in light of estimated human-induced warming to date”, while also providing tables with data for 0.3–1.1 °C of additional warming to highlight the potential effects of different temperature definitions and pre-industrial reference periods for estimates of remaining budgets (refs 17,18 in Millar et al.7). The difference between model-based globally complete surface air temperature (SAT) and globally incomplete combinations of blended air and sea surface temperature observations is important for quantifying climate impacts at low-temperature thresholds. This difference is larger over the historical period than in projected future changes under ambitious mitigation. Studies of impacts of 1.5 °C of warming should indeed acknowledge this difference, but it is relatively small for ambitious mitigation scenarios expressed relative to the present decade (less than 0.05 °C — difference between dark blue and purple lines in Fig. 2 of ref. 6). In their 2017 paper, Schurer et al.8 stated that “blended observational data sets ... will probably be those used to determine whether a temperature threshold has been reached”. Our use of global SAT projections (ref. 7, Fig. 1, Tables 1 and 2) means that budget estimates for thresholds of warming beyond the present decade are actually slightly underestimated relative to budgets under a blended metric, with the same being true for the AR5 budget estimates. It is important to understand differences in the definitions of global average temperature in mitigation and climate impact studies. However, the definition of warming in the context of the Paris Agreement is not informed solely by physical geoscience considerations9,10. Our paper estimated the outstanding carbon budget consistent with limiting the increase in global average temperature above pre-industrial levels to 1.5 °C, using a definition of present-day warming consistent with government-approved assessments that directly informed the Paris Agreement, while acknowledging that other interpretations were possible. We therefore stand by the central definition of warming used in our paper and its estimate of the remaining carbon budget. ❐

Short Black Carbon lifetime inferred from a global set of aircraft observations

Black Carbon (BC) aerosols substantially affect the global climate. However, accurate simulation of BC atmospheric transport remains elusive, due to shortcomings in modeling and a shortage of constraining measurements. Recently, several studies have compared simulations with observed vertical concentration profiles, and diagnosed a global-mean BC atmospheric residence time of <5 days. These studies have, however, been focused on limited geographical regions, and used temporally and spatially coarse model information. Here we expand on previous results by comparing a wide range of recent aircraft measurements from multiple regions, including the Arctic and the Atlantic and Pacific oceans, to simulated distributions obtained at varying spatial and temporal resolution. By perturbing BC removal processes and using current best-estimate emissions, we confirm a constraint on the global-mean BC lifetime of <5.5 days, shorter than in many current global models, over a broader geographical range than has so far been possible. Sampling resolution influences the results, although generally without introducing major bias. However, we uncover large regional differences in the diagnosed lifetime, in particular in the Arctic. We also find that only a weak constraint can be placed in the African outflow region over the South Atlantic, indicating inaccurate emission sources or model representation of transport and microphysical processes. While our results confirm that BC lifetime is shorter than predicted by most recent climate models, they also cast doubt on the usability of the concept of a “global-mean BC lifetime” for climate impact studies, or as an indicator of model skill.Aerosols: black carbon has short atmospheric residence timeA geographically wide set of aircraft measurements show that the lifetime of black carbon aerosols in the atmosphere is less than five days. Chemical transport simulations are key for assessing the climate impact of black carbon aerosols, but their evaluation has been limited by sparse observations. Marianne T. Lund, from the Center for International Climate Research in Oslo, Norway, and colleagues compare a wide range of aircraft black carbon concentrations from 2008–2017 with two chemistry transport models to confirm a short black carbon lifetime of less than 5 days. While this global mean lifetime applies over a wide geographical range, they also find important regional discrepancies, in particular in the Arctic. These findings caution against using a global-mean lifetime to diagnose the impact of black carbon on warming.